Related Rule
Practice Relating to Rule 10. Civilian Objects’ Loss of Protection from Attack
Belgium’s Teaching Manual for Soldiers states that objects occupied or used by enemy military forces are military objectives “even if these objects were civilian at the outset (houses, schools or churches occupied by the enemy)”. 
Belgium, Droit de la Guerre, Dossier d’Instruction pour Soldat, à l’attention des officiers instructeurs, JS3, Etat-Major Général, Forces Armées belges, undated, pp. 20–21.